Thanks for all the info.
I ordered an R5 and Control Ring adapter from Panamoz yesterday.
To re-cap, I have the 100-400 Mk2 and 1 1.4 tc Mk3.
Using the bare lens on my 7D2, I have a reach of 400mm x 1.6 = 640mm. I already crop heavily, sometimes up to 100% and, to me, the results seem quite acceptable. Using the lens and tc on the R5, I would have a reach of 560mm. The R5 sensor size lends itself to cropping; maybe up to 100%?
I value the versatility of the 100-400, particularly for insects and flowers. I can't see that the RF800mm offers this. I do not often uses hides and would not want to carry two long lenses despite the lightness of the RF11. Part of the attraction of the RF system and adaptor is the possibility to use existing glass.
Who knows when Covid restrictions might be lifted; let lone enough to allow us to safely go on foreign birding trips.
Our local nature reserve is currently submerged.
There are strong hints of a possible R7 APS-C mirrorless later this year.
Sans R5, I would be no worse off than I am now.
Bearing in mind the above, I wondered if the R5 for my purposes might be a beautiful pig in a poke.
After yet more agonising, I equivocate no longer and have cancelled R5 - excellent response from Panamoz, by the way.
Let's see what an R7 might have to offer.
Colin